Integrated circuits (ICs) are commonly made in the well-known complementary insulated gate field effect transistor (FET) technology known as CMOS. CMOS technology and chip manufacturing advances have resulted in a steady decrease of chip feature size to increase on-chip circuit switching frequency (circuit performance) and the number of transistors (circuit density). In what is typically referred to as scaling, device or FET features are shrunk to shrink corresponding device minimum dimensions including both horizontal dimensions (e.g., minimum channel length) and vertical dimensions, e.g., channel layer depth, gate dielectric thickness, junction depths and etc. Shrinking device size increases device density and device performance, as well as reduces device-operating conditions, i.e., chip and correspondingly, device supply voltages and voltage swings. Consequently, as a result of scaling otherwise seemingly neglectable device-to-device variations (e.g., length, width, threshold and etc.) have caused serious design problems, especially in signal critical circuits such as memory sense amplifiers.
A typical CMOS circuit includes paired complementary devices, i.e., an n-type FET (NFET) paired with a corresponding p-type FET (PFET), usually gated by the same signal. Since the pair of devices have operating characteristics that are, essentially, opposite each other, when one device (e.g., the NFET) is on and conducting (ideally modeled as a closed switch), the other device (the PFET) is off, not conducting (ideally modeled as an open switch) and, vice versa. So, for example, a CMOS inverter is a series connected PFET and NFET pair that are connected between a power supply voltage (Vdd) and ground (GND).
An ideal static random access memory (SRAM) cell includes a balanced pair of cross-coupled inverters storing a single data bit with a high at the output of one inverter and a low at the output of the other. A pair of pass gates (also ideally, a balanced pair of FETs) selectively connects the complementary outputs of the cross-coupled inverter to a corresponding complementary pair of bit lines. A word line connected to the gates of the pass gate FETs selects the cell, connecting the cell contents to the corresponding complementary pair of bit lines. During a read, each cell on the selected word line couples its contents to its corresponding bit line pair through NFET pass gates. Since the bit line pair is typically pre-charged to some common voltage, initially, the internal (to the cell) low voltage rises until one of the bit line pairs droops sufficiently to develop a small difference signal (e.g., 30 mV). A simple ideal sense amplifier or, sense amp, is a matched pair of cross-coupled common-source devices connected between a bit line pair and an enable source line. Device imbalances in matched cell devices or the matched sense amp pair can unbalance the pair to seriously erode the sense signal margin and even cause sense amplifier errors.
Leakage currents can cause an inadequately balanced sense amplifier to self-trigger. Leakage from high floating-device body-voltages may cause large offset voltages scattered unevenly in SOI devices that may trigger the sense amplifier prematurely, latching false data. Similarly, SRAM cells can become instable from such leakage and cell performance may degrade. Robust sensing techniques have been developed to subside to deal with device variability. However, evaluating such a sensing technique requires providing a variable differential signal that may be slewed within a range of interest. This may be done for an entire SRAM data path (e.g., macro or chip), for example, by varying array/cell supply voltage and determining read and write failing points. Unfortunately, this only gives an overall figure of merit for the data path. Because circuits such as sense amps, of necessity, are very sensitive, it is not particularly helpful in evaluating such robust sensing circuits. It has been especially difficult to evaluate circuit response to a small voltage differential in such a sense circuit isolated from SRAM cells, i.e., outside of a data path. Consequently, it is difficult to characterize and evaluate state of the art SRAM cell sensing circuits.
Thus, there is a need for circuit that reliably test and characterize SRAM cell sensing circuits and especially for simple and in-line test and characterization circuits that test small signal circuits to assist in deciding the merit of new sensing circuits.